When cutting, for example, metal with a CO.sub.2 laser, an assist gas jet is directed by a nozzle onto the metal being cut.
Throughout this specification the expression "assist gas" is intended to embrace a gas which, during a thermic cutting process, is actively involved in the cutting process in that its primary function is either to react with or to provide a shielding atmosphere to protect the material being cut from reaction; and to provide kinetic energy to the molten material being removed during cutting. For the avoidance of doubt the expression "assist gas" is not intended to cover gas that has a modulating effect on the power source of a thermic cutter. In the case of laser cutting, the assist gas is essential for blowing out the molten metal and debris as the laser beam passes over the metal. As aforesaid the assist gas may be inert and thereby protect the metal sheet from oxidation or contamination. Alternatively, the assist gas may be reactive and thus act exothermically with the metal being cut thereby releasing heat such that the cutting operation is enhanced in terms of faster cutting speeds.
Currently, most laser cut metal sheets show pronounced striations which are caused due to the periodic melting effect which occurs coupled with the uneven ejection of the molten metal and debris by the assist gas.
UK application, Publication No. 2194190, describes a method or and an apparatus for intermittently welding or cutting material using a supersonic plasma arc torch which is controlled by modulating the gas flow to the torch at a frequency equal to a desired rate of repetition of cutting or welding actions. A high gas flow rate results in a plasma jet of greater power density and diameter than a lower gas flow rate. In other words, the gas has a modulating effect on the power density of the prime heat source of the torch. This document does not relate to the continuous thermic cutting of material by the modulation of an assist gas.